Lies My ATIS Told Me

When you get right down to it, and you are getting close to the ground, precise knowledge of local weather is the one critical determinant as to whether your three-hour instrument flight is going to have a happy ending, or become a cliffhanger at decision height. Weather reporting is often the single most important factor affecting the critical few minutes of an instrument approach (or even a VFR approach), as well as our decision-making process. As life would have it, the last Wednesday in May taught me that in no uncertain terms. What follows is a lesson in precise weather for precision approaches…

THE PLOT THICKENS
I was coming into Boston’s Logan airport on the ILS for runway 4R. The ATIS was reporting 300 scattered, 3800 broken, with good visibility. I was on a medical flight for AirLifeLine, bringing a burn patient into Signature for pickup and transport to the Shriner’s burn unit. We were already late because I’d had to wait two hours for below-minimums weather to lift that morning, before I could get into Manassas, Virginia, to pick up the patient. The tower had made that oft-heard request for a ‘good rate‘ on final, and there were airliners in the conga line right behind us. I was flying a Cherokee in visual conditions, and with 300 scattered, I wasn’t too worried about getting behind the airplane at 130 knots, so that’s what I gave him. I’d been monitoring the ATIS since before it had gone from ‘kilo‘ to ‘lima‘, and things were looking good. The grunt of appreciation I got back on the radio felt just like he’d patted me on the back; and that felt good, too. After I’d intercepted the glide slope, though, I looked ahead and all I could see was a carpet of white. Something was wrong.

…AND SO DID THE CLOUDS
And something stayed wrong. At 200 feet, I was still inside a milk bottle, and I went around. I made a point of telling the controller, in effect ‘look, I don’t know where your ATIS observation was, but out here, there’s sure as heck no 300 scattered.‘ The controller wasn’t happy with me, and it wasn’t because of my non-standard phraseology. However, after the next two airliners also shot missed approaches, I imagine that he was much less annoyed. But I digress… We did make it in on a subsequent approach. Later on, I did some homework, and here’s what I found out:

AUTOMATED WEATHER 101
You probably don’t need me to give you an intro course about the four levels of Automated Weather Observation System (AWOS), which is more of a state-funded affair and doesn’t give as complete a picture as the federally funded and more sophisticated Automated Surface Observing System (ASOS) installations. Still, it’s worth pointing out that most AWOS sites don’t give precipitation type (though an AWOS-3P does), or information on thunderstorm activity (AWOS-3T), but the ASOS usually does (though with its precipitation threshold of around 0.01 inch per hour, it doesn’t detect light rain or drizzle). You probably already know that a METAR‘s ‘A02‘ means it can tell the difference between rain, freezing rain, or snow, and that an ‘AUTO‘ means there isn’t a human observer on duty at that site. You were most likely aware that the ceiling resolution is 100 feet, and that an ASOS can’t count higher than three (separate cloud layers, that is). And you probably also know that both AWOS and ASOS give winds relative to true North … except when the information is broadcast over ATIS (and via telephone) or it goes to Flight Watch, in which case it’s converted to magnetic.

To be fair, two large studies done several years ago actually found that agreement between human and automated observations was great — in VMC — but the disparity does go up in MVFR and IFR weather.

Inside Information: Interestingly, the more recent study in 1997 found that automated systems reported ceilings below 200 and visibility under a half mile about 50% more often than did human observers. Plus, ASOS visibility differences were pronounced whenever visibility was six miles or less, or there was showery precipitation. (This isn’t too startling considering that the automated reading comes from a ten-minute average inside less than a cubic meter of air at one fixed point, and one might also intuitively expect that rain can scatter a return pulse like so much chaff.)

AUTOMATED WEATHER 301
Over 1000 US airports now have either an AWOS, an ASOS, or at some of the bigger ones of course, an ATIS (which always comes from an ASOS). Wind speed and direction, temperature, and altimeter information almost always work great. We get slightly lower minimums, and advance notice for important factors affecting what runway to use and what kind of performance we can expect. But the inherent difference between a human observer and an automated system rears its ugly head when it comes to ceiling and visibility, especially when your destination is experiencing a ‘meteorological discontinuity‘, whether it is geographically induced, or not.

Why (hold on tight): Automated systems use a ‘fixed location, time averaged‘ value, and human observers of course will always ‘eyeball it‘ with the ‘fixed time, spatial averaging‘ method. The gallium arsenide laser ceilometer looks up from one location near the touchdown zone during ‘detection cycles‘ that can each be as brief as 12 seconds (during which thousands of pulses are sent), and averages these over 30 minutes, with a double-weighted average over the last 10.

Translation: You basically get a ‘downwind,‘ ‘after-the-fact‘ report of sky conditions. That’s what I’d gotten that day. Of course, there could very well have been 300 scattered at the touchdown zone, but with a three degree glide slope, the touchdown zone is over three thousand, eight hundred feet away from where you are at decision height. That’s more than enough room for convective weather, the proverbial ‘hole in the clouds‘, or a fog bank to pull the rug out from under you. (This last one is particularly critical for you to remember if you’re flying into an airport near the water like Westerly, Rhode Island, Block Island, Nantucket, Massachusetts, or Boston, as I was.)

BACK TO LOGAN
As it turns out, at Logan they have several ceilometers around the field (a wise option when an airport is near a large body of water), as well as a full-time weather observer from Taunton to supplement the ASOS readings (which we hear as an ATIS). Somewhere around a third of ASOS facilities are at airports with part-time facilities (Level C) or else they have full-time observers (Levels B and A) to augment or correct ASOS observations. The problem was, that observer was in Logan’s old ‘Terminal A‘, which was (as I was told by a controller who was there at the time) in the clouds, anyway. Are you beginning to get the picture?

Remember: It can take several minutes for any sudden change in ceiling or visibility that occurred during the last 10 minutes to even show up. So whenever things are changing, or marginal, or worse, listen for at least 10 minutes. It’s always a good idea to listen in as far out as you can.

Inside Information: Whenever it’s bright enough to make you reach for the sunglasses and it’s also hazy, chop the visibility report in half. The machine is using the more efficient ‘forward scatter‘ to basically look into its own headlights — your eyes only see half as far through the murk’s backscatter, which is much less penetrating.

Ask ATC if anyone ‘went missed.‘ This will warn you in advance if ceilings and/or visibilities are lower than reported. Last, but not least, follow standard procedures regarding when you can land and when you shouldn’t, and always be prepared to go around.

BOTTOM LINE: The weather at your destination will decide if in fact you’ll be landing there. Getting the most accurate weather is not as simple as listening to ATIS. Be prepared for the unexpected. Know the limitations of reporting equipment. Be prepared to go somewhere else.